1. Field of the Invention
The present invention relates to a method and to a device intended for seismic prospecting or monitoring of an underwater subsurface zone using seismic receivers coupled with the bottom of the water body.
2. Description of the Prior Art
It is well-known to carry out seismic exploration or monitoring operations in a subsea zone, which comprise transmission of seismic waves in the subsurface formation, reception and recording of the waves reflected by the formation discontinuities by means of seismic receivers coupled with the surface of the bottom, and processing the data acquired so as to form seismic images.
U.S. Pat. No. 4,870,625 and WO-9,923,510 describe seismic prospecting methods using a seismic cable or streamer, a first end of which is connected at the surface to a boat or a buoy, long enough for a part at the opposite end to rest on the sea bed. Seismic receivers such as geophones or hydrophones are associated with this part and coupled with the sea bed. The receivers are connected by lines inside the streamer to a surface acquisition equipment.
It is well-known from U.S. Pat. No. 4,692,906 and 5,189,642 to lower to the sea bed seismic units comprising seismic receivers coupled to the sea bed, an electronic equipment for acquisition and recording of the seismic signals picked up in response to the actuation of a seismic source towed underwater, and means for bringing the seismic units back up to the surface at the end of the exploration sessions.
French Patent 2,774,775, filed by the assignee, describes a method for seismic prospecting or monitoring of a subsea reservoir, wherein one or more seismic emission units comprising one or more seismic sources associated with a power supply are lowered onto the sea bottom, each unit being connected to the surface by a multifunction umbilical. Seismic receivers can also be coupled with the bottom surface.
The device according to the invention allows seismic exploration of a subsurface zone immersed in a water body. The device comprises at least one seismic signal acquisition unit comprising a streamlined part or spire provided with a housing for at least one seismic receiver and a data collection module for data corresponding to seismic signals received by the receivers, each acquisition unit moving downwards under the effect of gravity to the bottom of the water body and to attach thereto so as to couple the seismic receivers with the subsurface formation, the device also comprising a positioning device for each acquisition unit at the bottom and a surface recovery for the seismic data collected by each acquisition unit.
The positioning devices comprise for example an acoustic transponder associated with each acquisition unit and an acoustic ranging set associated with a surface installation for detecting the relative position of each acquisition unit, whether a short-baseline acoustic positioning device associated with a vehicle equipped with satellite positioning or several acoustic transponders associated with one or more surface buoys provided with satellite positioning.
According to an embodiment, each acquisition unit comprises a first transmission set for transmission of the seismic data acquired, and the surface recovery for recovery of the acquired seismic data comprise a vehicle equipped with a second transmission set suited to communicate with the first transmission set.
According to another embodiment, each acquisition unit comprises a first transmission set for transmission of the acquired seismic data, the surface recovery of the acquired seismic data comprising a data collection device provided with a second transmission set suited to communicate with the first transmission set, this device being connected by a cable to a surface installation and brought underwater at a certain distance from each acquisition unit, long enough for transmission of the data.
According to another embodiment, the seismic data recovery comprise removable floats associated with each acquisition unit, suited, on command from a surface installation, to bring back up to the surface at least part of each acquisition unit, such as the collection module.
According to another embodiment, each acquisition unit comprises storage elements for the acquired seismic data, and the removable floats are associated with the storage elements and suited, on command from a surface installation, to bring them back to the surface.
According to another embodiment, each acquisition unit comprises a barrel containing several memory cartridges for the seismic data acquired, and the removable floats are associated with at least one memory cartridge and suited, on command from a surface installation, to bring the memory cartridge back to the surface.
The method according to the invention comprises:
installing the device as defined above;
carrying out seismic exploration operations with emission of waves by an underwater seismic source, reception by the receivers of each acquisition unit of the waves reflected by discontinuities of the subsurface formation in response to the waves emitted in the formation, collection of the data corresponding to the waves received by the receivers, and recovery at the surface of the seismic data acquired by each acquisition unit.
The method comprises for example:
installing at the bottom of the water body at least one seismic signal acquisition unit comprising a streamlined part or spire provided with a housing for at least one seismic receiver and a data collection module for data corresponding to seismic signals received by the receivers, each unit being suited to move downwards under the effect of gravity to the bottom of the water body and adhere to the bottom so as to couple the seismic receivers with the subsurface formation;
determining the position of each acquisition unit installed at the bottom;
carrying out seismic operations with emission of waves by an underwater seismic source, reception of the waves reflected by discontinuities of the subsurface formation in response to the waves emitted in the formation, collection of the data received by the receivers of each acquisition unit; and
recovering at the surface the seismic data acquired by each acquisition unit.
Recovery, at the surface, of the seismic data acquired by each acquisition unit comprises either transfer to the surface of at least part of each acquisition unit, or transmission of the data to a surface installation by means of a vehicle or of a passive device brought near to the acquisition units.
The method and the device can be applied for example for marine seismic prospecting, repetitive marine seismic prospecting in a subsurface formation under development, etc.